yes,i agree about using tank rather then pond.BUT,arowana that kept in tank will almost surely have dropped eye(s),soon or later,compare to an aro that live in pond that usually have lower percentage to have dropped eyes.don't know why too..
That would be because arowanas in a tank will generally be up closer to our eye level and then they would have more things to look at below as people/animals walk past. So by looking down a lot, they will develop droop eye. Whereas in a pond, most of the people/animals walking by would be above them. Not to mention insects and what not would be above the water surface. Thus, by looking up a lot they are less likely to get droop eye.